Our program represents a systematic, mutational approach to transfer RNA structure-function relationships. It starts with selections for specific genetic alterations (missense suppressors) which have significance in vivo, moves on to the identification of the molecular lesion and aims at a greater understanding of precisely what aspects of the structure of certain tRNA are involved in their different functions and interactions. It calls for: (1) the utilization of missense codons, mainly at position 211 of the alpha subunit of tryptophan synthetase, where the behavior of (virtually) all amino acids is known, to obtain the conversion of specific tRNAs to new coding and amino acid accepting specificities; (2) genetic, physiological, and biochemical characterization of the mutants; (3) isolation and purification of the altered tRNAs by appropriate techniques such as column chromatography, polyacrylamide gel electrophoresis, etc., and determination of the change in nucleotide sequence and/or nature and extent of base modification; (4) correlation of altered sequence with altered functions and other changes in tRNA structure and molecular interactions.